1.l Field of the Invention
The present invention relates to a three-dimensional position measurement method for projecting measurement light onto an object to measure a position of an irradiated portion of the object and to an apparatus used for the three-dimensional position measurement.
2. Description of the Related Art
Non-contact three-dimensional input devices called rangefinders are used for data entry into CAD systems or CG systems, three-dimensional measurement of various types of articles or bodies, visual recognition for robots and others. The three-dimensional input device of this type projects measurement light onto an object to photograph the same, and then to output three-dimensional position information of plural points on the object. On this occasion, three-dimensional positions of the respective points are determined by operations based on the triangulation method.
The operations based on the triangulation method use the following three pieces of information.    (1) Projection angle of measurement light    (2) Acceptance angle of reflected light from an object    (3) Positional relationship between a projection optical system and a receiving optical systemIn order to obtain the correct coordinates of a certain point on the object, with respect to the point, it is necessary that the items (1) and (2) correspond to each other one-to-one. The case where only measurement light reflected from the point on the object is received is the one-to-one relationship.
There are often instances when, in addition to measurement light reflected from a certain point on an object (hereinafter referred to as “primary reflected light”), measurement light that was reflected from the point and further reflected from another point on the object (hereinafter referred to as “secondary reflected light”) is received. In such a case, two acceptance angles correspond to one projection angle. Unless an operation is performed using the acceptance angle of the primary reflected light, which is true information, of the two acceptance angles, the correct coordinates cannot be determined.
Japanese unexamined patent publication No. 2-184705 is a related art document that is directed to problems of the secondary reflected light as described above. The document describes a method using a property that a rotation direction of circular polarized light is reversed every time when the circular polarized light is reflected. More specifically, the document describes a method of projecting the circular polarized light on an object as measurement light and passing the light reflected from the object through a quarter wavelength plate and a polarizing plate in this order. The quarter wavelength plate converts the circular polarized light to linear polarized light. At this time, the quarter wavelength plate makes the polarization direction of primary reflected light orthogonal to the polarization direction of secondary reflected light. The polarizing plate is arranged so as to transmit the primary reflected light mainly, thereby to provide the effect of attenuating the secondary reflected light. According to the disclosure of the document, a received light signal where secondary reflected light components are removed is generated by using only one received light signal obtained by photoelectric conversion of the light that has passed through the polarizing plate. On this occasion, the primary reflected light components and the secondary reflected light components are distinguished from each other on the premise that the intensity of the primary reflected light is higher than that of the secondary reflected light.
U.S. Pat. No. 5,444,537 describes a method of projecting a plurality of light beams modulated into different patterns to determine the intensity ratio of the beams, and thereby to remove secondary reflected light components from a received light signal.
With the conventional method of projecting circular polarized light, correct measurement results are not always obtained, unless the intensity of the primary reflected light is sufficiently higher than that of the secondary reflected light. In practical cases, even if the secondary reflected light is attenuated using the combination of the quarter wavelength plate and the polarizing plate, the intensity of the secondary reflected light may be greater than that of the primary reflected light or there may be little difference in the intensity between the secondary reflected light and the primary reflected light. For example, there may be instances when the primary reflected light is weak diffused light and the secondary reflected light is strong light that was regularly reflected from an object twice. In such a case, the primary reflected light components rather than the secondary reflected light components may be removed from the received light signal by mistake. The removal of the primary reflected light components results in calculation of the incorrect coordinates.
Meanwhile, with the method of projecting a plurality of light beams modulated into different patterns, a projection optical system becomes complicated and expensive.